The purpose of this project is twofold: first, to demonstrate that the active transport processes of the retinal pigment epithelium (PE) can help determine the ionic milieu in the retinal and choroidal space; secondly, that the composition and oxygen tension of the ionic milieu can affect the active transport and fluid secretion across the epithelium. In other words, our purpose is to demonstrate the homestatic ability of the PE. Most of these experiments will utilize the isolated frog PE-choroid or retina PE-choroid preparation. Mechanisms of active and passive transport have been previously worked out in this preparation. In these experiments the tissue will be mounted in specially designed chambers so that a variety of measurements can be made; biochemical, electrophysiological, ion flux, O2 consumption and volume flow. The first step will be to show that the isolated PE-choroid consumes O2 and transports fluid and that the rate of these processes can be severely reduced by selective inhibition of various active transport processes. The composition and osmolarity of the transported fluid will be measure as a function of bathing solution composition. This should indicate the homeostatic ability of the isolated PE. A likely mechanism of fluid secretion involving c-AMP will also be investigated. Another major series of experiments will utilize the isolated frog retina PE-choroid preparation. In these experiments the fluid flow across the preparation and the O2 consumption in the retina (O2 microelectrode) will be measured as a function of light induced retinal activity as a function of chroidal O2 tension. The volume flow experiments shown the retinal influence on active PE transport and the O2 experiments show the effect of choroidal O2 tension and PE consumption rate on retinal function. Together these findings would demonstrate the homeostatic ability of the pigment epithelium.